Integral circular wastewater treatment plant

ABSTRACT

The plant includes circular outer and inner walls, first and second radial partitions across the intermediate volume spaced to form a first arcuate clarifier portion and a second arcuate portion, first and second covered aeration zones with gas and liquor passages therebetween, and means for uniformly distributing oxygenated liquor around the clarifier inner wall for radial flow toward the outer wall.

United States Patent LaClair et al.

1 1 June 17, 1975 1 INTEGRAL CIRCULAR WASTEWATER 2.027.370 1/1936 Currie210/201 x TREATMENT PLANT 2,506,927 5/1950 Kelly .1 210/197 2.901.1148/1959 Smith et a] 210/205 X 175] Inventors: Louis Maynard LaClair,Grand 2.963.430 12/1960 Schreiber 210 4 island, N.Y.; John Ruben3.733.264 5/1973 Spector et al 210/7 McWhirter, Westport, Conn; WilliamLawrence Ross, FOREIGN PATENTS OR APPLICATIONS s o n, w. Va. 1.037 s438/1966 United Kingdom 210/199 [731 Assignee: Union Carbide Corporation,New

York Primary Examiner-Roy Lake [22] Filed: Apr. 2, 1973 AssistantExaminer-Craig R. Feinberg [2'] A I N 347 398 Attorney, Agent, orF1'rm-S. J. Hultquist 1521 US. (:1. 210/199; 210/205; 210/220; 1ABSTRACT [5 H 1m CI The plant includes circular outer and inner walls,first [58] Fie'm 10/35 14 and second radial partitions across theintermediate 210]]5 256 volume spaced to form a first arcuate clarifierportion 525 527 f and a second arcuate portion, first and second coveredaeration zones with gas and liquor passages therebetween. and means foruniformly distributing oxygenated liquor around the clarifier inner wallfor [56] UNITE S S; /I I ES ;:Z I ENTS radial flow toward the outerwall,

584.736 6/1897 Goodhue 210/200 14 Claims, 17 Drawing Figures 52 C 660539 A A an PATENTEDJIJN 1 7 I975 SHEET FIG.4

PATENTEDJUNH ms 3,890,231

LIQZET Length of Liquid Flow Pufh,(Ft)

PATENTEIJJUN 17 ms p 1:51

m ill F s. I? A 0 1 l 1 l 1 l l Circumferential Arc Length of Clorifier(Degrees) 8 9 O 2 3 l PATENTEDJUN 1 7 I975 mm 1 1 Length of Liquid FlowP0th,(Ft.)

0 1 t I l 1 360 320 280 240 200 I60 lZO Circumferential Arc Length ofClorifier (Degrees) FIG. I5

Length of Liquid Flow Puth,(Ft.)

I l l 0 l t l l l 330 I am 290 270 250 230 2&0

Circumferential Arc Length of Clarifier (Degrees) INTEGRAL CIRCULARWASTEWATER TREATMENT PLANT BACKGROUND OF THE INVENTION This inventionrelates to a method of and apparatus for treating wastewater by aerationwith at least 50 percent oxygen in an integral circular plant.

In areas where small flows of wastewater require treatment it isdesirable to employ integral plants, i.e. plants in which all componentsare enclosed in a single outer wall. The cost of material andfabrication are lower for a relatively small integral wastewatertreatment plant than for a plant comprising physically separateelements. Moreover, integral plants are compact and require a small landarea for installation; such a plant also has a potential for much moresimplified overall design as compared to a non-integrated facility.

Notwithstanding the requirement of being relatively small, the integralplant must maintain the desired level of wastewater treatment, i.e. thegeometries of the constituent segments must promote good performance.For example, the mixing or aeration segments must promote efficient flowpatterns and distribution of contained liquor; the clarifier mustpromote a low BOD content 4 effluent water and thickening of sludgeunderflow.

The prior art has made extensive use of circular plants for relativelysmall wastewater flows as they offer several advantages over otherconfigurations such as rectangular. By providing a minimum perimeter tocross sectional area ratio, circular design tends to minimize materialrequirements for fabrication of the integral plant while promoting ahighly efficient component arrangement. Additionally, construction costsmay be less in some instances for circular geometries than for othershapes, as for example in concrete fabrication.

The prior art has employed biological treatment processes in smallcircular plants, primarily because of their applicability to a widevariety of wastewaters and effluent requirements and comparatively lowcapital cost. The major biological treatment process in commercial useis based on activated sludge, in which wastewater is mixed in anaeration zone with oxygencontaining gas and the activated sludge. Thelatter consists essentially of aerobic organisms which in the presenceof dissolved oxygen, absorb and assimulate the biochemically oxidizableorganic content (BOD) of the wastewater, converting the organic materialto forms which can readily be separated from the purified water in theclarification zone. Under normal conditions the organisms multiplyrapidly in the aeration zone and when the requisite period of BODconversion is complete, the mixed liquor is settled in a clarifier zoneand the purified effluent decanted to receiving waters. Sludge iswithdrawn from the bottom of the clarifier zone with part thereof beingrecycled to the aeration zone to maintain effective biological action onthe influent wastewater.

Until very recently atmospheric air has been the sole source of oxygenin activated sludge plants. But in recent years this systcm has beenvastly improved by the use of high purity oxygen gas as the oxidant in aseries of closed rectangular tanks, preferably with staging of gas andliquor from tank to tank inthe manner described in the US. Pat. Nos,3,547,813, 3,547,8l4 and 3,547,815 all to J. R. McWhirter. The highpurity oxygen aerated systems offer important advantages over airaerated plants as for example higher levels of biological action oninfluent wastewater therefore smaller aeration tanks.

The operation of clarification is greatly influenced by the type ofaeration employed. Clarifiers in the activated sludge process have twofunctions: They must provide an effluent with a low level of suspendedsolids and must also thicken sedimentary solids and provide a sludge ofsufficient concentration to maintain effective biological action in theaeration zone. The efficiency of the clarifier in performing these twofunctions depends largely on the physical nature of the solids in theliquor discharged from the aeration zone and here again the oxygenaeration process has distinct advantages over air aeration systems. Thelatter produces typically small-sized fragile, relatively unflocculatedsolids particles which do not settle well in the clarifier. Moreover,the settled sludge possesses a high specific volume as for examplemeasured by the Sludge Volume Index (SVI) so that because of the poorsettling characteristics and compactibility, a clarifier processing airaerated sludge must be comparatively large in size to insure adequateperformance. Oxygen aeration systems by contrast produce sludge withsuperior settling characteristics, i.e. higher settling velocities,(lower SW) and better dewatering ability.

In sizing clarifiers the dual functions of clarification and thickeningmust be separately considered and an overall area chosen whichaccomodates both requirements. lt is further necessary to develop aclarifier design which is free from stagnant areas or shortcircuitingflows. This is accomplished by providing a geometric form without sharpcorners or regions inaccessible to the liquor flow, and uniform fluidflow patterns within the clarifier. Although the latter characteristicis primarily insured by distributing the influent liquor as uniformly aspossible over the entire cross sectional area of the clarifier, it isalso necessary to provide liquor flow patterns within the vessel whichpermit sufficient liquor residence time for sedimentation to occur. Itis also desirable to provide a liquor flow path in the clarifier whichbrings the influent strength to a relatively quiescent state and thusminimize fluid velocities within the bulk fluid volume.

To effectively use the entire area provided in the clarifier, the lengthof the liquor flow path must be at least equal to and at peak flowconditions preferably identical with the path length necessary forsedimentation. If the sedimentation path is shorter than the actual pathprovided for liquor travel then distribution of solids will occur overonly part of the clarifier area. Under these circumstances the clarifierhas been over designed and the integral plant is larger than necessary.lf the sedimentation path is longer than the actual path provided forliquor travel then a gross loss of solids may occur in the clarifiereffluent. Unfortunately, the prior art air aeration integral circularplants with arcuate clarifier zones require the zone to extend aroundthe entire periphery of the outer wall, i.e. 360, for the liquor flowpath length to be at least equal to the sedimentation path length. Thatis, foreshorting of the clarifier arc to less than the fullcircumference causes the sedimentation path length to exceed the actualliquor flow path length and substantially reduce the solidsliquidseparation in the clarifier. This means that only the central portion ofan air aerated integral circular 3 plant is available as the aerationzone and the plant must be sized on the basis of the required aerationzone volume. The result of this severe limitation is a relatively largeplant to process a given wastewater flow rate.

An object of this invention is to provide an improved method of andapparatus for biological treatment of wastewater in an integral circularplant.

Another object of this invention is to provide an acti' vated sludgesystem employing high purity oxygen aer ation for relatively lowwastewater flow rates in an integral circular plant which issubstantially more compact than rectangular configuration plants.

SUMMARY This invention relates to a method of and apparatus for treatingwastewater by aeration with at least 50 percent oxygen in an integralcircular plant,

It has been unexpectedly discovered that in an oxygen aerated wastetreatment system integral circular plant with a peripheral clarificationzone, the clarifier arc length may be foreshortened to as little as 90without causing the sedimentation path length to exceed the actualliquid flow path length. This means that the remaining peripheralportion of the plant may be employed for other purposes, as for example,aeration. digestion of the activated sludge, and chlorination of theclarifier effluent. To achieve the desired wastewater treatment withoxygen gas so as to effectively utilize the relatively expensive oxygen(compared to air), reduce the BOD content of the effluent to anacceptable level and obtain activated sludge having the aforementionedsuperior settling characteristics, it is necessary in the practice ofthis invention to employ at least two separate oxygen aeration zoneswith the oxygen depleted gas from the first zone being transferred tothe second aeration zone and the first oxygenated liquor also beingtransferred to the second aeration zone. At least one aeration zone isprovided in an arcuate portion of the plant, which portion is availabledue to the unique foreshortened clarifier arcuate portion. As usedhereinafter the term arcuate portion" refers to an enclosed part of theintegral circular wastewater treatment plant bounded on the outside by acircular tank outer wall, on the inside by the circular inner wall, andon the ends by radially extended end walls with the circumferentiallength of the concentric inner and outer walls serving to define arclengths of less than 360.

The arc length of the clarifier zone of this plant may be as low as 90and still provide an actual liquid flow path length at least as long asthe sedimentation path length. From a theoretical standpoint, it shouldbe possible (if desired) to employ a fully extended 360 clarifier zonein the integral circular waste treatment plant of this invention, sinceat this arc length the sedimentation path length based on idealizedconditions is still less than the actual provided flow path length.However for such an arc length in a fixed diameter plant, the inner toouter wall distance in the clarifier is a minimum since the inner walldiameter must be increased to provide the entire required volume for atleast two aeration zones. With such narrow clarifier portion widths thehydraulic effects associated with clarifier inlet and outletdisturbances become increasingly important and adversely effectclarifier performance. For this reason the clarifier arcuate portionshould not exceed 330 so as to obtain suitably long inner to outer wallwidths and satisfactory hydraulic conditions at the inner wall whereoxygenated liquor is introduced and the outer wall where the clarifiedeffluent is discharged.

In the integral circular wastewater treatment plant of this inventionthe oxygenated liquor from the final aeration zone is directed to meansfor uniformly distributing the same in the first arcuate portion of theintermediate volume between the inner wall segment thereof. That is, theoxygenated liquor flows radially outward from the inner wall to theouter wall and the actual liquor flow path length is therefore theradial distance between the walls. The final aeration zone is preferablywithin the circular inner wall as this location most readily accomodatesthe necessary radial flow in the clarifier arcuate portion. That is,restricted opening(s) may be provided in the inner wall to achieve thede sired flow of oxygenated liquor from the final aeration zone to thefirst arcuate clarifier portion of the intermediate volume.

A third radial partition may extend across the intermediate volumebetween and joined at opposite ends to the inner and outer walls withinthe second arcuate portion so as to form another arcuate portion withinthe intermediate volume. The second aeration zone may then be locatedwithin the second arcuate portion and separated from the first aerationzone by the third radial partition.

The integral circular wastewater treatment plant of this inventionoffers substantial advantages over presently employed air aeratedcircular plants. By way of example and on the basis of a wastewater flowrate of 1 million gallons per day, the present plant requires only 47percent of the ground area required by the air aerated plant. Theinstant plant also is substantially more compact than a rectangular typeplant also employing oxygen aeration as for example described in theaforementioned McWhirter US. Pat. No. 3,547,815. Again based on awastewater flow rate of l MGD, this circular plant would require anouter wall area of only 31 percent of the outer wall area required by arectangular plant based on identical process conditions, correspondingto a wall length of 87 ft. for the circular plant and 280 ft. for therectangular plant.

More specifically, the wastewater treatment apparatus of this inventioncomprises a circular tank outer wall, a circular inner wall concentricwith and spaced from the outer wall forming an inner volume and anintermediate volume between the inner and outer walls such that theratio of the inner wall radius (R,) to the outer wall radius (R isbetween 0.25 and 0.70. A first radial partition extends across theintermediate volume between and joined at opposite edges to the innerand outer walls, and a second radial partition also extends across theintermediate volume between and joined at opposite edges to the innerand outer walls. The second partition is spaced from the first radialpartition so as to form a first arcuate portion of the intermediatevolume bounded by segments of the inner and outer walls, comprisingbetween 90 and 330 of their respective circumferences. A second arcuateportion comprises the remainder of the intermediate volume.

First fluid mixing and recirculation means are provided within the outerwall in a first part other than the first arcuate portion and a coverencloses this part posi tioned over the first fluid mixing andrecirculation means to form a first aeration zone. First passage meansintroduce oxygen gas in the first aeration zone,

and second passage means introduce feed wastewater and activated sludgeto the first aeration zone.

Second lluid mixing and recirculation means are provided within theouter wall in a second part other than the first arcuate portion, and asecond cover encloses the second part positioned over the second fluidmixing and recirculation means to form a second aeration zone.

First gas interzone transfer means are spaced from the oxygen gas firstpassage means for discharging oxygen-depleted gas from the firstaeration zone and introducing same to the second aeration zone as theoxygencontaining gas therefor. First liquor interzone transfer means areprovided for discharging first oxygenated liquor from the first aerationzone and introducing same to the second aeration zone for mixing thereinwitht the oxygen-containing gas. Gas vent means are spaced from thefirst gas interzone transfer means for discharging oxygen-furtherdepleted gas from the second aeration zone. Liquor passage meansdischarge second oxygenated liquor from the second aeration zone andmeans are provided for uniformly distributing oxygenated liquor in thefirst arcuate portion of the intermediate volume around the inner wallsegment for radial flow across said first arcuate portion. Trough meansaround the outer wall segment upper part of the first areuate portionare employed for discharging clarified water, and means are included forcollecting and removing activated sludge from the bottom part of thefirst arcuate portion and returning at least part of the sludge to thesecond passage means to the first aeration zone.

This invention also includes an improved method for waste watertreatment. In a circular air aerated plant, the clarifier arc length maynot be reduced below 360 in order to prevent the sedimentation pathlength from grossly exceeding the actual liquor path length. In such aplant, if oxygenated liquor were discharged from a central clarifier forradial outward flow towards the outer wall, the distribution area wouldbe very large (due to the fully extended 360 wall) and the radialvelocity of the liquor would be very low. However, because the settlingvelocity of air-aerated activated sludge is inherently low, the radialliquid velocity needs to be decreased to a value much lower than itsinlet value in order to achieve good sedimentation. Within the geometricconstraints of the overall package plant, this is not possible for theair system. Only a small degree of liquid expansion is possible, so thatthe radial liquid velocity is not substantially reduced.

In the method of this invention, the aeration volume, in addition tobeing significantly smaller than in the air system. is divided into atleast two zones. This means that the central circular final aerationzone is small in size. and provides a small. localized distribution areafor oxygenated liquor introduction into the arcuate clarification zone.The localized distribution area promotes high radial liquid velocitiesat the inner smaller diameter inlet are. but, because of the long radial(actual) liquor flow path provided and the higher characteristicsettling velocity of oxygenated sludge. sufficient expansion oftheliquor is realized to achieve good sedimentation The sedimentation pathis thus contained by the actual provided radial flow path. because ofthe beneficial radial expansion of liquid in the clarifier.

More specifically. the waste water treatment method of this invention isby aeration with at least 50'? oxygen gas in the presence of recycledactivated sludge for biological oxidation in at least two coveredaeration zones wherein the oxygen feed gas. waste water and activatedsludge are mixed and one fluid is simultaneously recirculated in a firstaeration zone, oxygen partially depleted gas and partially oxygenatedliquor from the first aeration zone are each separately introduced to asecond aeration zone for continuous mixing and fluid recirculationtherein, and the further oxygenated liquor from the final aeration zoneis separated into effluent water and activated sludge in a clarificationzone with at least part of the sludge being returned to the firstaeration zone as said recycled sludge. The improvement comprises: (a)mixing said oxygen feed gas, waste water and recycled activated sludgein an arcuate first aeration zone; (b) mixing oxygen partially depletedgas and partially oxygenated liquor in a circular final aeration zone;(c) flowing the further oxygenated liquor radially across an arcuateclarification zone of between and 330arc length from an inner smallerdiameter inlet arc to an outer concentric larger diameter liquideffluent discharge are at radial velocities and volumetric flow ratessuch that V,;/\/, is between 0.1 and 0.5, wherein diameter liquideffluent discharge arc.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings FIG. I is a schematicflow sheet of a wastewater treatment system in which the presentinvention may be practiced.

FIG. 2 is an isometric view of an integral circular wastewater treatmentplant incorporating the invention.

FIG. 3 is a plan view of a plant similar to the FIG. 2 plant showing afirst arcuate aeration zone, a second center aeration zone. an arcuateaerobic disgestion zone and a l90arcuate clarifier.

FIG. 4 is a schematic plan view of the FIG. 3 plant showing the fluidsflows.

FIG. 5 is an elevation view of the FIG. 3 plant taken in cross sectionalong line AA.

FIG. 6 is another elevation view of the FIG. 3 plant taken in crosssection along line BB.

FIG. 7 is still another elevation view of the FIG. 3 plant taken incross section along line C-C.

FIG. 8 is a schematic plan view of an alternative integral circularwastewater treatment plant similar to FIG. 4 but with a 295arcuateclarifier and showing the fluid flows.

FIG. 9 is a schematic plan view of still another alternative integralcircular wastewater treatment plant with three arcuate aeration zones, afourth center aeration zone and 99arcuate clarifier, and showing thefluid flows.

FIG. 10 is a plant view of an additional integral circu lar wastewatertreatment plant showing two arcuate aeration zones, an arcuatechlorination zone, a central aerobic digestion zones, and a222clarifier.

FIG. 1] is an elevation view of the FIG. 10 plant taken in cross sectionalong line A-A.

FIG, 12 is another elevation view of the FIG. 10 plant taken in crosssection along line BB.

FIG. 13 is still another elevation view of the FIG. 10 plant taken incross section along line C-C.

FIG. 14 is a graph showing the clarifier performance of an air aeratedcircular plant processing wastewater of 250 mg/l BOD in an aeration zonewith total suspended solids concentration (MLSS) of 2200 mg/l.

FIG. 15 is a graph showing clarifier performance in an oxygen aeratedcircular plant processing wastewater of 250 mg/l BOD at total suspendedsolids concentration (MLSS) of 5000 mg/l.

FIG. 16 is a graph showing clarifier performance of an air aeratedcircular plant processing wastewater of 686 mg/l BOD in an aeration zonewith total suspended solids concentrated (MLSS) of 2200 mg/I.

FIG. 17 is a graph showing clarifier performance in oxygen aeratedcircular plant processing wastewater of 686 mg/l at total suspendedsolids concentration (MLSS) of 6500 mg/l.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring more specifically to thedrawings, FIG. I is a schematic flow sheet of a wastewater treatmentsystem in which the present invention may be practiced, and includescircular tank outer wall 20 and circular inner wall 21 concentric withand spaced from outer wall 20 forming an inner volume 22 and anintermediate volume 23 between the inner and outer walls. The ratio ofthe inner wall radius R. to the outer wall radius R is between 0.25 and0.70. Wastewater is introduced through conduit 24 to first aeration zone25, recycled sludge is also introduced to this zone through conduit 26and at least 50 percent by volume oxygen gas is introduced to the samezone through conduit 27 having control valve 28 therein.

As illustrated, first aeration zone is an arcuate portion of the plantand enclosed by overhead cover 29. First fluid mixing and recirculationmeans are provided in first aeration zone and comprise rotatableimpeller 30 positioned at the liquor surface, sub-surface propeller 31is positioned beneath the impeller, common shaft 32 joining the impellerand propeller and extending through cover 29, and motor drive means 33connected to shaft 32 for rotation thereof. Propeller 31 performs thefunction of continuously sub-surface mix ing of the fluids, whileimpeller 30 throws sheets of liquor outwardly against the gas andperforms the continuous recirculation function (of liquor). If outertank 20 is shallow, surface impeller 30 may perform both functions andsub-surface propeller 31 may be unnecessary. Other mechanical means forfluid mixing and recirculation may be employed, for example a systemincluding a sub-surface propeller, a gas sparger, and a gasrecirculation pump joined to the gas space above the liquor and beneaththe coverv As discussed and illustrated in US. Pat. No. 3,547,815 to J.R. McWhirter, the gas sparger may be positioned at the bottom end ofahollow vertical rotatable shaft, with the propeller also mounted on theshaft above the sparger. The gas recirculation pump may be mounted onthe cover with its inlet side joined to the gas space by a conduitthrough the cover. The pump discharge side is joined to the top end ofthe rotatable shaft for recirculation of oxygen gas to the sparger andthence into the liquor.

The sludge is recycled to first aeration zone 25 at rate so as tomaintain the desired total solids concentration (MLSS) as for example6000 mg/l and volatile suspended solids concentration (MLVSS) as forexample 4500 mg/l. Broad suitable ranges for these parameters are 40008000 mg/l MLSS and 3000-6000 mg/l MLVSS. The food-to-biomass ratio maybe in the range of 0.5-155 gm BOD /day X gm MLVSS, for example about0.68. The recycled sludge concentration (MLSS) is in the range ofl5,00050,000 mg/l. The oxygen gas is introduced in sufficient quantityto maintain dissolved oxygen concentration (DO) in the mixed liquor of4-8 mg/l and for example 6 mg/l. Oxygen control valve 28 may beautomatically adjusted in response to sensed oxygen vapor pressure inthe overhead gas space as monitored by sensor 34 and transmitting means35.

The fluids in first aeration zone are mixed for the desired period, e.g.45 minutes liquid residence time, and the resulting first oxygenatedliquor is discharged through opening 36 in inner wall 21 to secondaeration zone 37. Simultaneously, oxygen-depleted gas from the firstaeration zone 25 is passed through first gas innerzone transfer means 38to second zone 37 and the fluids are again mixed in this zone by secondfluid mixing and recirculation means comprising propeller 39, im' peller40, shaft 41, and motor 42. Second aeration zone 37 is enclosed by cover43, and the operating parameters may be substantially the same aspreviously discussed in connection with the first aeration zone 25.After the desired liquor residence time, as for example 45 minutes, thesecond oxygenated liquor is discharged from second zone 37 throughliquid passage means 44 and to clarifier zone 45 bounded by inner wall21, outer wall 20 and first and second radial partitions (notillustrated in FIG. 1). Oxygen-further depleted gas is discharged fromsecond aeration zone 37 through gas vent means 46. It will be noted thatthe gas discharge means from each of first and second aeration zones 25and 37 are spaced from the gas inlet means to these respective zones soas to avoid bypassing gradient.

The oxygenated liquor entering clarifier zone 45 flows radially acrossthis zone towards outer wall 20 and the solids settle from the liquidduring this radial path. Trough means 47 extend around the outer wallend segment of the clarifier 45 for discharging clarified watertherefrom through conduit 48. As used herein, trough means 47 includeany means for collecting and removing clarified water as for example theillustrated open conduit, or a submerged perforated conduit. Means areprovided for collecting and removing activated sludge from the bottompart of clarifier 45 and returning at least part of same to firstaeration zone 25 through the aforementioned means 26. The aforementionedmay include sludge pickup heads 49 each positioned near the bottom ofclarifier zone 45, and vertical conduits 50 each with a lower end joinedto a sludge pick-up head and an outer end terminating above sludgetrough 51. Bridge 52 extends across and above the clarifier zone andsupports the pick-up headvertical conduit assembly. Mechanical drivemeans (not illustrated) are provided for moving the bridge in an arcuatepath repetitively around the clarifier between the two radialpartitions. Pump means 99 are joined to conduit 50 for drawing sludgetherethrough.

In the FIG. 2 embodiment, the first aeration zone 25 is a part of thesecond arcuate portion comprising the remainder of the intermediatevolume other than the first arcuate clarifier portion 45. This firstaeration zone 25 has cover 29 and motor 33 supported by the cover andemployed for driving the first fluid mechanical mixing and recirculationmeans. The second aeration zone 37 is within inner wall 21 and enclosedby cover 43 supporting motor 42 for driving the second fluid mixing andrecirculation means. The first arcuate clarifier portion 45 is uncoveredand the bridge assembly 52 for supporting the sludge pick-upheadvertical conduit assembly is illustrated. An arcuate aerobicdigestion portion 53 adjoins the first aeration zone 25 and separatedfrom clarifier portion by first radial partition 54. First aeration zone25 is separated from clarifier 45 by second radial 56 partition and thetwo cov ered arcuate portions (first aeration zone 25 and aerobicdigestion zone 53) are separated from each other by third radialpartition 57 extending to the floor of the plant between outer and innerwalls and 21. Fluid mixing and recirculation means are provided inaerobic digestion zone 53, and driven by motor 58 positioned on cover55.

FIG. 3 is a plan view of an embodiment of this invention similar to theHO. 2 plant showing arcuate first aeration zone 25. central secondaeration zone 37, arcuate aerobic digestion zone 53, and l90arcuateclarifier zone 45. Also included is arcuate chlorination zone 6]. Feedwastewater is introduced to first aeration zone through inlet conduit24, stop gate 62 and bar screen 63. Oxygen feed gas is introducedthrough overhead conduit 27. and sludge recycled from clarifier 45 byoverhead circular trough 51 extending around the outer periphery ofinner wall 21 between first radial partition 54 and second radialpartition 56. Slight positive gas pressure is maintained under the firstaeration zone cover to prevent back mixing of gas between joiningaeration zones 25 and 37. The first and second aeration zones areprovided with pressure relief valves 64 and 65 respectively. Thepreviously described mechanical fluid mixing and recirculation systempromotes transfer of oxygen to the mixed liquor in the first aerationzone 25. the biomass assimulates and metabolizes BOD leading toproduction of CO water and additional microorganisms. Oxygen is consumedand the gas purity in the space above the mixed liquor decreases. Thepartially oxygenated liquor and oxygen depleted gas are transferred tocentral second aeration zone 37 for further fluids mixing andrecirculation. Oxygen further depleted gas is vented from second aeration zone 37 through conduit 46 and the oxygenated liquor flows radiallyoutward across clarifier 45. Sludge collecting bridge and overheadmanifold 52 move in an arcuate path between first and second radialpartitions 54 and 56, with air lift blower 66 joined to manifold 51 asthe needed suction for withdrawing sludge from the clarifier floor anddischarging same into trough 51. The non recycled sludge is transferredby trough 51 to aerobic digestion zone 53, also provided with mechanicalfluid mixing and recirculation means similar to the corresponding meansused in the first and second aeration zones. The aforementionedmixing-recirculation means and drive motor 58 may be supported onbridges 59. Oxygen gas may be supplied to the aerobic digestion zonefrom an external source or at least in part from the second aerationzone vent. The clarified water flows over effluent weir 67 into trough68 extending around the inner perimeter of the clarifier zone 45. Thiseffluent flows into chlorination zone 61 for disinfection and is thendischarged through effluent conduit 69.

Solids digestion is carried out in zone 53 with supernatant liquid beingreturned if desired to first aeration zone 25 through stilling well 70.Waste sludge is discharged from digestion zone 53 through conduit 71.

FIG. 4 shows the fluid flows in the aforedescribed integral circularwastewater treatment plant. In brief the water flows sequentiallythrough first aeration zone 25, second aeration zone 37 to clarificationzone 45 where the oxygenated liquor is separated into activated sludgeand clarified water. The latter is directed to chlorination zone 61 (ifemployed) and discharged to receiving water. A portion of the activatedsludge is recycled to first aeration zone 25 and the balance transferredto aerobic digestion zone 53 for further aeration. The supernatantliquid may be returned to first aeration zone 25 through stilling well70. The oxygen gas entering first aeration zone 25 is partially consumedtherein. oxygen depleted gas discharged through means 38 to secondaeration zone 37 and the further oxygen depleted gas is vented throughmeans 46.

FIG. 5 is an elevation view of the FIG. 3 plant taken in cross sectionalong line A-A showing the activated sludge collection and removalassembly in greater detail. More particularly a series of sludge pick-upheads 49a-d are transversely spaced across first arcuate clarifierportion 45 and positioned near the floor 72, being respectivelysupported by hollow shafts SOa-d for flow therethrough to overheadsludge trough 51. Air lift blower 66 mounted on second aeration zonecover 43 is joined through conduits 73 and 74 to each of verticalconduits 75ad joined to the lower end of shafts 50a-d respectively. andprovides the needed suction for drawing sludge upwardly therethrough.Motor 76 moves sludge pick-up bridge assembly 52 around the clarifierarcuate path. The oxygenated sludge flows from second aeration zone 37into clarifier zone 45 through liquid passage means 44 associated withinner wall 21. The latter comprises vertical projections 77 extendingfrom floor 72 and spaced on either side of inner wall 21 extendingdownwardly to a position near but spaced from the floor 72 with a narrowgap therebetween.

FIG. 6 is an elevation view of the FIG. 3 plant taken in cross sectionalong line B-B showing the inner wall portion separating first arcuateaeration zone 25 and central second aeration zone 37. Circular opening38 in the upper portion of inner wall 21 permits restricted flow ofoxygen depleted gas from the first to the second aeration zone, whileslot opening 36 in the lower portion of inner wall permits restrictedflow of first oxygenated liquor from the first to the second zone.Horizontal slots 78 at the liquor level are provided for passage of foamfrom the first to the second zone.

FIG. 7 is an elevation view of the FIG. 3 plant taken in cross sectionalong line CC showing second radial partition 56 separating firstarcuate clarifier portion 45 and chlorination zone 61. In thisparticular embodiment the liquid level in clarifier zone (indicated byhigher horizontal dotted line) is above the liquid level in thechlorination zone 61 (indicated by lower horizontal dotted line).Clarified water flows over outlet weir 67 into trough 68 around theperimeter of clarifier 45 and within outer wall 20, and intochlorination zone 61. Sludge trough 51 is shown positioned against theouter side of inner wall 21. Second oxygenated liquor from secondaeration zone 37 flows around vertical projections 77 from clarifierfloor 72 and beneath inner wall 21 into clarifier 45 for radial outwardflow there across as previously described.

FIG. 8 is a schematic plan view of an alternative integral circularwastewater treatment plant differing from the previously describedembodiments in that the first aeration zone 25 comprises the entiresecond arcuate portion and the first arcuate clarifier portion 45comprises the balance of the intermediate volume between outer wall andinner wall 21. In this arrangement there are no waste treatment zonesother than aeration and clarification and the clarifier arc length maybe on the order of 295. The ratio of clarifier cross sectional area toaeration volume may be about 0.192, and the ratio of inner wall radiusR, to the outer wall radius R may be about 0.382.

FIG. 9 is a schematic plan view of still another alternative integralcircular wastewater treatment plant especially suited for treatment ofextremely high BOD content wastewater. Four aeration zones are providedand perferably arranged for cocurrent staged flow of oxygen containinggas and wastewater through the four stages. More particularly threearcuate aeration zones, a fourth center aeration zone, a 99 arcuateclarifier and an arcuate aerobic digestion zone are included. Asillustrated in FIG. 9, first arcuate aeration zone is separated fromarcuate clarifier zone 45 by first radial partition 54 and from secondarcuate aeration zone 37 by third radial partition 57. Second arcuateaeration zone 37 and third arcuate aeration zone 78 are separated byfourth radial partition 79. Third arcuate aeration zone 78 and arcuateaerobic digestion zone 53 are separated by fifth radial partition 80,and the opposite end of digestion zone 53 separated from arcuateclarifier zone 45 by second radial partition 56. Fourth aeration zone 81is located in the center portion of the plant within inner wall 21. Withthe exception of arcuate clarifier portion 45 the entire plant isenclosed by a cover. The fluid inner connections between the variouszones may be identical to those previously discussed and illustrated inFIGS. 6 and 7. By way ofillustration. the ratio of clarifiercross-sectional area to aeration volume may be about 0.0316 and theratio of inner wall radius R, to the outer wall radius R2 is about0.467.

FIG. 10 is a plan view of an additional embodiment wherein aerobicdigestion zone 53 is located in the ceritral part ofthe plant withininner wall 21. First aeration zone 25 is within the second arcuateportion and separated on one side from second arcuate aeration zone 37by third radial partition 85 and from radial chlorination zone 61 on theother side by fourth radial partition 86. Second arcuate aeration zone37 is separated from first arcuate clarifier portion 45 by first radialpartition 54. Oxygenaled liquor from second aeration zone 37 enterslaunder 87 extending around the outer periphery of inner wall 21 withinthe arcuate clarifier portion 45 and overflows uniformly andcontinuously into the clarifier for radial flow there across. Theclarified effluent water flows through trough 68 into chlorination zone6] separated from the clarifier zone by second radial partition 56.Settled solids in clarifier 45 are moved into troughs at the extremeends of the clarifier by a scraper assembly mounted on bridge 52. Thelatter travels by means of tracks on the inner and outer walls aroundthe clarifier arcuate portion and is driven by reciprocal drive means 76terminated at each end by bridge stops 88 so that the scraper is activein both directions. The troughs are sloped toward the inner wall 21 anda portion of the collected sludge is recycled through conduits 89 havingpumps 90 therein to arcuate first aeration zone 25. The balance of thesludge is directed to central aerobic digestion zone 53 and waste sludgedischarged therefrom through conduit 71 passing through second aerationzone 37.

FIG. 11 is an elevation view of the FIG. 10 plant taken in cross sectionalong line A-A and showing the bridge-sludge scraper assembly. Scraper91 extends transversely between outer wall 20 and inner wall 21,horizontally aligned slightly above plant floor 72 and is supported frombridge 52 by arms 92. Bridge 52 moves around the arcuate clarifierportion on rollers 93.

FIG. 12 is another elevation view of the FIG. 10 plant taken in crosssection along line B-B showing the sludge return assembly. The settledsolids accumulate in sludge trough 95 at the lower end first radialpartition 54 and are drawn upwardly through a vertical section ofconduit 89 by sludge return pump 90. The unrecycled portion is directedthrough branch conduit 96 to aerobic digester 53 for further aerationtherein. The horizontal dotted line indicates the liquor level in secondaeration zone 37.

FIG 13 is still another elevation view of the FIG. 10 plant taken incross section along line C-C showing the second aeration zone fluidsmixing and recirculation system and the waste sludge discharge. Conduit71 extends from central aerobic digestion zone 53 through inner wall 21and radially across floor 72 from arcuate second aeration zone 37,emerging through outer wall 20.

FIGS. 14-17 compare actual liquor flow paths and sedimentation pathlengths for clarifiers of air aerated circular wastewater treatmentplants and oxygen aerated circular plants in accordance with thisinvention. The sedimentation path length has been determined on theassumption of a uniform distribution of aerated Iiquor over a verticalcross section of area adjacent the inner clarifier wall and a uniformradial distribution of liquid velocities in the FIGS. l417 graphs. CurveA represents the sedimentation path length and Curve B represents theactual liquor flow path length.

FIG. 14 shows the clarifier performance of an air aerated plantprocessing wastewater of 250 mg/l BOD with total suspended solidsconcentration of 2200 mg/1, whereas FIG. 15 shows clarifier performancein a two step oxygen aerated plant processing wastewater of the same BODstrength but at higher total suspended solids concentration in theaeration zone of 5000 mg/l. It will be apparent from a comparison ofthese curves that in the air aerated plant of FIG. 14 the actual liquidflow path length only approaches the sedimentation path length with afully extended 360 clarifier configuration and that any decrease in theclarifier arc length will prevent the plant from effectively producing alow solids content effluent. In marked contrast, with the presentinvention clarifier arc lengths as low as 260 may be employed with theliquor flow path length exeeeding the sedimentation path length. therebypermiting effective solids separation in the clarifier.

FIGS. 16 and 17 show clarifier performance of respectively air andoxygen aerated circular plants processing wastewater of 686 mg/l BODwith total suspended solids concentration (MLSS) of 2200 mg/l (airaerated zone) and 6500 mg/l (each of the two oxygen aerated zones).

As in the case of the FIG. 14 lower strength wastewater. FIG 16 showsthat with higher BOD content waste water. air aerated circular plantsalso require a fully ex' tended 360 clarifier configuration, i.e.sedimentation path length curve A is above actual liquor flow pathlength curve B over the entire range of circumferential arc length ofclarifier. However, FlG. 17 shows that circumferential arc lengthsgreater than about 180 are sufficient to provide effective separation ofsolids from water and a purified effluent. It is significant to notethat the air aerated circular plants represented by FIGS. 14 and 16 donot have the flexibility for the inclusion of treatment zones such asthe afore described aerobic digestion and chlorination zones included inplants based on this invention.

Within the broad range of 90 to 330 clarifier arc length. it ispreferred to employ l80 to 300 clarifier arc lengths when processingrelatively low strength waste water ofless than 300 mg/l, and 90 to 240clarifier arc lengths when processing relatively high strength wastewater of greater than 300 mg/l. Also, it is preferred to employ 180 to330 clarifier arc lengths when contact stabilization is practiced in thecircular integral plant of this invention, i.e. a relatively smallaeration stage of short liquor residence time to remove supernatantliquid and partially concentrate the solids for further aeration.

The aforementioned preferred ranges reflect the general tendency foroptimum clarifier arc length to decrease as the ratio ofaeration zonearea to arcuate clarifier area increases, the larger aeration zone areabeing required to treat higher BOD concentrations.

Table l summarizes suitable process conditions for the oxygen aeratedintegral circular plant of this invention in comparison with air aeratedplants for treatment of typical municipal waste water.

Also in the preferred practice of this invention. the ratio of clarifierarcuate cross-sectit'mal area to the total .leration zone \oltlme may berelated to low and high waste water BOD feed concentrations. Forinfluent waste water BOD concentrations of less than about 300 mg/l, thearcuate clarifier area/aeration zone ratio is preferably between about0.10 ft and 0.25 ft. while the comparable ratios for air aeratedcircular plants are in the range of about 0.02 ft to 0. [0 ft. Forinfluent waste water BOD concentrations above 300 mg/l, the arcuateclarifier area/aeration zone ratio is preferably between about 0.05 ftand 0.ll ft". whereas the comparable ratios for air aerated circularplants are in the range of about 0 to 0.04 ft"l Summarizing, for lowstrength waste waters, the arcuate clarifier preferably occupies acomparatively large portion of the total plant area whereas for highstrength waste water, the arcuate clarifier preferably occupies a relatively smaller fraction of the total plant area.

Table ll compares the aeration zone cross-sectional areas and theclarifier areas required for air aerated circular plants and oxygenaerated circular integral plants with arcuate clarification portions,all based on l X l0 gal/day waste water flows.

The above table shows the oxygen system to have a substantially smalleraeration volume than the air systern (liquid depths are constant. l2ft.) at typical operating conditions. As an example, at 200 mg/l BOD,the aeration volume of the typical oxygen system is about 25 percent ofthe corresponding air system volume. The reason for this disparity isshown in Table I. The air system is able to attain only a very lowconcentration level of active biological solids (MLVSS), typically900-2600 mg/l, and thus must supply extremely large aeration tankage inorder to provide the long liquor retention times necessary forreasonable BOD removals. The oxygen system. however, having inherentlyhigher biological solids levels, maintains correspondingly higher levelsof biological assimilation and is able thereby to operate with muchsmaller aeration chambers.

The wastewater treatment capability of an aeration system can bedescribed in terms of an operational range of organic loadings to thesystem, expressed as lbs. BOD applied/day/IOO ft of aeration zonevolume. Air systems typically operate at values of 3060 lbs.BOD/day/lOOO ft", while oxygen system of this invention can operate at60300 lbs. BOD/day/l000 ft. For any given BOD loading then, the oxygenaeration system will be smaller in size than the corresponding airsystem. It has previously been indicated that the ratio of the innerwall radius (R to the outer wall radius (R is between 0.25 and 0.70.

If R,/R exceeds 0.70, the intermediate volume becomes too narrow toaccommodate arcuate aeration zones with uniformly good mixingcharacteristics. i.e. the zones would be excessively long relative totheir width. Also. the arcuate clarifier portion would be so narrow asto develop aberant flow phenomena, with a prohibitively low actualliquor radial flow path to the

1. Wastewater treatment apparatus comprising: a. A circular tank outer wall; b. A circular inner wall concentric with and spaced from said outer wall forming an inner volume and intermediate volume between said inner and outer walls, such that the ratio of the inner wall radius (R1) to the outer wall radius (R2) is between 0.25 and 0.70; c. A first radial partition extending across said intermediate volume between and joined at opposite edges to said inner and outer walls; d. A second radial partition extending across said intermediate volume between and joined at opposite edges to said inner and outer walls, being spaced from said first raidal partition so as to form a first arcuate portion of said intermediate volume bounded by segments of said inner and outer walls comprising between 90* and 330* of their respective circumferences, and a second arcuate portion comprising the remainder of said intermediate volume; e. First fluid mixing and recirculation means within said outer wall in a first part other than said first arcuate portion and a cover enclosing said first part positioned over said first fluid mixing and recirculation means to form a first aeration zone; f. First passage means for introducing oxygen gas in said first aeration zone; g. Second passage means for introducing feed wastewater and thickened activated sludge to said first aeration zone; h. Second fluid mixing and recirculation means within said outer wall in a second part other than said first arcuate portion and a second cover enclosing said second part positioned over said second fluid mixing and recirculation means to form a second aeration zone; i. First gas interzone transfer means spaced from first passage means (f) for discharging oxygen-depleted gas from said first aeration zone and introducing same to said second aeration zone as the oxygen-containing gas therefore; j. First liquor interzone transfer means for discharging first oxygenated liquor from said first aeration zone and introducing same to said second aeration zone for mixing therein with said oxygen-containing gas; k. Gas vent means spaced from first gas interzone transfer means (i) for discharging oxygen-further depleted gas from said second aeration zone; l. Liquor passage means for discharging second oxygenated liquor from said second aeration zone; m. Means for uniformly distributing oxygenated liquor in said first arcuate portion of said intermediate volume around the inner wall segment for radial flow across said first arcuate portion for both clarification of said liquor and thickening of activated sludge therein; n. Trough means around the outer wall segment upper part of said first arcuate portion for discharging clarified water thereform; and o. Means for collecting and removing thickened activated sludge from the bottom part of said first arcuate portion and returning at least part of said sludge to said second passage means (g).
 2. Apparatus according to claim 1 wherein said inner volume within said circular inner wall comprises said second aeration zone.
 3. Apparatus according to claim 1 with a third radial partition extending across said intermediate volume between and joined at opposite edges to said inner and outer walls within said second arcuate portion, third fluid mixing and recirculation means within said outer wall in a third part other than said first arcuate portion, a third cover enclosing said third part positioned over said third fluid mixing and recirculation means to form a third aeration zone, secoNd gas interzone transfer means joined to gas vent means (k) for introducing said oxygen - further depleted gas from said second aeration zone to said third aeration zone, second liquor interzone transfer means joined to liquor passage means (1) for introducing second oxygenated liquor to said third aeration zone, other liquor passage means for discharging third oxygenated liquor from said third aeration zone and joining uniform distribution means (m) and other gas vent means spaced from said second gas interzone transfer means for discharging oxygen - still further depleted gas from said third aeration zone.
 4. Apparatus according to claim 1 with a third radial partition extending across said intermediate volume between and joined at opposite edges to said inner and outer walls within said second arcuate portion, third fluid mixing and recirculation means within said outer wall in a third part other than said first arcuate portion, a third cover enclosing said third part positioned over said third fluid mixing and recirculation means to form a thickened sludge digestion zone, passage means for introducing oxygen gas to said sludge digest zone, sludge transfer means joined to sludge return means (o) for transferring the unreturned thickened sludge to said sludge digestion zone, means for discharging digested thickened sludge from said sludge digestion zone and still other gas vent means spaced from the oxygen gas introduction passage means for discharging oxygen - depleted gas from said sludge digestion zone.
 5. Apparatus according to claim 1 wherein ratio of the inner wall radius (R1) to the outer wall radius (R2) is between 0.30 and 0.60.
 6. Apparatus according to claim 1 wherein a rotatable impeller positioned at the liquor surface, a subsurface propeller positioned beneath said propeller a common shaft joining said impeller and said propeller and motor drive means connected to said shaft comprise each of said first and second fluid mixing and recirculation means.
 7. Apparatus according to claim 1 wherein said means (o) for collecting and removing thickened activated sludge comprises a multiplicity of sludge pickup heads each positioned near the bottom of said first arcuate portion and spaced between said inner and outer walls, a multiplicity of conduits each with a lower end joined to a sludge pick-up head and an upper end in flow communication with said passage means (g) for introducing thickened activated sludge to said first aeration zone, a bridge extending across and above said intermediate volume and supporting the pick-up head - multiple conduit - assembly, mechanical drive means for moving said bridge in an arcuate path repetitively around said first arcuate portion between said first and second radial partitions, and pump means joined to said overhead manifold for moving thickened sludge therethrough.
 8. Apparatus according to claim 1 wherein a third radial partition extends across said intermediate volume between and joined at opposite edges to said inner and outer walls within said second arcuate portion to form said second aeration zone.
 9. Apparatus according to claim 4 with a fourth radial partition extended across said intermediate volume between and joined at opposite edges to said inner and outer walls within said second arcuate portion, spaced from and between said first and third radial partitions, and a chlorination zone within said outer wall in a fourth part other than said first arcuate portion, with means for introducing the clarified water discharged from said first arcuate portion to said chlorination zone, and means for discharging disinfected effluent from said chlorination zone.
 10. Apparatus according to claim 9 wherein said second arcuate portion comprises said sludge digestion zone third part between said first and fourth radial partitions, said first aeration zone first part between said third and fourth radial partitions, and said chlorination zone fourth part between sAid second and third radial partitions, and said inner volume within said circular inner wall comprises said second aeration zone second part.
 11. Wastewater treatment apparatus comprising: a. A circular tank outer wall; b. A circular inner wall concentric with and spaced from said outer wall forming an inner volume and intermediate volume between said inner and outer walls, such that the ratio of the inner wall radius (R1) to the outer wall radius (R2) is between 0.25 and 0.70; c. A first radial partition extending across said intermediate volume between and joined at opposite edges to said inner and outer walls; d. A second radial partition extending across said intermediate volume between and joined at opposite edges to said inner and outer walls, being spaced from said first radial partition so as to form a first arcuate portion of said intermediate volume bounded by segments of said inner and outer walls comprising between 90* and 330* of their respective circumferences, and a second arcuate portion comprising the remainder of said intermediate volume. e. First fluid mixing and recirculation means within said outer wall in a first part of said second arcuate portion and a cover enclosing said first part positioned over said first fluid mixing and recirculation means to form a first aeration zone; f. First passage means for introducing oxygen gas in said first aeration zone; g. Second passage means for introducing feed wastewater and thickened activated sludge to said first aeration zone; h. Second fluid mixing and recirculation means within said inner wall in said inner volume and a second cover enclosing said inner volume positioned over said second fluid mixing and recirculation means to form a second final aeration zone; i. First gas interzone transfer means spaced from first passage means (f) for discharging oxygen-depleted gas from said first aeration zone and introducing same to said second aeration zone as the oxygen-containing gas therefor; j. First liquor interzone transfer means for discharging first oxygenated liquor from said first aeration zone and introducing same to said second aeration zone for mixing therein with said oxygen-containing gas; k. Gas vent means spaced from first gas interzone transfer means (i) for discharging oxygen-further depleted gas from said second aeration zone; l. Liquor passage means for discharging second oxygenated liquor from said second aeration zone; m. Means for uniformly distributing oxygenated liquor in said first arcuate portion of said intermediate volume around the inner wall segment for radial flow across said first arcuate portion for both clarification of said liquor and thickening of activated sludge therein; n. Trough means around the outer wall segment upper part of said first arcuate portion for discharging clarified water therefrom; and o. Means for collecting and removing thickened activated sludge from the bottom part of said first arcuate portion and returning at least part of said sludge to said second passage means (g).
 12. Wastewater treatment apparatus comprising: a. A circular tank outer wall; b. A circular inner wall concentric with and spaced from said outer wall forming an inner volume and intermediate volume between said inner and outer walls, such that the ratio of the inner wall radius (R1) to the outer wall radius (R2) is between 0.25 and 0.70; c. A first radial partition extending across said intermediate volume between and joined at opposite edges to said inner and outer walls; d. A second radial partition extending across said intermediate volume between and joined at opposite edges to said inner and outer walls, being spaced from said first radial partition so as to form a first arcuate portion of said intermediate volume bounded by segments of said inner and outEr walls comprising between 90* and 330* of their respective circumferences, and a second arcuate portion comprising the remainder of said intermediate volume; e. First fluid mixing and recirculation means within said outer wall in a first part in said second arcuate portion and a cover enclosing said first part positioned over said first fluid mixing and recirculation means to form a first aeration zone; f. First passage means for introducing oxygen gas in said first aeration zone; g. Second passage means for introducing feed wastewater and thickened activated sludge to said first aeration zone; h. A third radial partition extending across said intermediate volume between and joined at opposite edges to said inner and outer wall within said second arcuate portion; i. Second fluid mixing and recirculation means within said outer wall in a second part in said second arcuate portion and a second cover enclosing said second part positioned over said second fluid mixing and recirculation means to form a second aeration zone; j. First gas interzone transfer means spaced from first passage means (f) for discharging first oxygen-depleted gas from said first aeration zone and introducing same to said second aeration zone as the oxygen-containing gas therefor; k. First liquor interzone transfer means for discharging first oxygenated liquor from said first aeration zone and introducing same to said second aeration zone for mixing therein with said oxygen-containing gas; l. Gas vent means spaced from first gas interzone transfer means (i) for discharging second oxygen-depleted gas from said second aeration zone; m. Liquor passage means for discharging second oxygenated liquor from said second aeration zone; n. Third fluid mixing and recirculation means within said inner wall in said inner volume and a third cover enclosing said inner volume positioned over said third fluid mixing and recirculation means to form a final aeration zone; o. Third passage means for introducing oxygen partially-depleted gas from a preceding aeration zone to said final aeration zone; p. Fourth passage means for introducing partially oxygenated liquor from a preceding aeration zone to said final aeration zone; q. Gas vent means for discharging oxygenfinally depleted gas from said final aeration zone; r. Liquor passage means for discharging finally oxygenated liquor from said final aeration zone; s. Means for uniformly distributing finally oxygenated liquor in said first arcuate portion of said intermediate volume around the inner wall segment for radial flow across said first arcuate portion for both clarification of said liquor and thickening of activated sludge therein; t. Trough means around the outer wall segment upper part of said first arcuate portion for discharging clarified water therefrom; and u. Means for collecting and removing thickened activated sludge from the bottom part of said first arcuate portion and returning at least part of said sludge to said second passage means(g).
 13. Apparatus according to claim 1 wherein said first aeration zone is in a first part in said second arcuate portion and said inner volume within said circular inner wall comprises the said second and final aeration zone.
 14. Apparatus according to claim 1 wherein said inner volume within said inner wall comprises a final aeration zone. 